1. Field of the Invention
The present invention relates to a printer connected to a computer. A laser beam printer is usually connected to the computer, since the laser beam printer is a high-speed printer and is suited to deal with print information outputted from the computer. The present invention relates specifically to a laser beam printer which can print both of format information and print information in an overlaying condition.
2. Description of the Prior Art
In a laser beam printer, a photosensitive drum is line-scanned by a laser beam modulated according to print information, and a latent image formed on the photosensitive drum is developed to print a desired pattern. Information to be printed includes a fixed form of background information (namely, a format) such as seen in a slip, table, or the like and variable information. If the background information and variable information are processed in the same manner by a processor, the amount of processing to be performed is large and moreover the processing is complicated. Accordingly, the format is processed separately from the variable information.
U.S. Pat. No. 4,125,829 discloses a method of processing a format and variable information in different manners, in which a plurality of formats are stored in a memory in the form of digital data, a selected one of the formats is read out of the memory to be combined with variable print information, and the combined information thus formed is used in modulating a laser beam.
Another method is known which is shown in FIG. 1. A laser beam from a laser 1 is modulated by a modulator 2 in accordance with print information, and then deflected by a scanner 3 so as to scan the surface of a photosensitive drum 4. On the other hand, a format is printed off in a film 5, which is moved by a film driving device 6 in accordance with the rotating speed of the photosensitive drum. The image on the film 5 is projected on the photosensitive drum 4 by means of a lamp 10.
The scanning operation of the laser beam is delayed by a time required to move that part of the photosensitive drum 4, on which the image on the film 5 is projected, to the scanning position, in order that respective latent images of the format and variable print information overlap each other. Thereafter, the latent images are developed by a processor 8 to form an image on the photosensitive drum 4, and the image thus formed is transferred to printing paper 9.
In the former method, however, a great deal of effort is required to prepare digital data for expressing a format, a large amount of processing has to be carried out to read out the format, and moreover a memory for storing the format is required. For these reasons, a printer for carrying out this method becomes high in cost.
In the latter method, the above-mentioned shortcomings are eliminated, but the lamp 10 different from the laser 1 is needed, in addition of the film 5 and film driving device 6.
Both of these methods are disadvantageous in that an inexpensive printer cannot be obtained.